In glass manufacturing, glass is generally made by melting a batch of raw glass materials in a refractory lined furnace. The furnace may be heated by hydrocarbon burners, by electricity, or by a combination of burners and electricity. Electrically heated glass furnaces include a refractory lined melting chamber for holding a body of molten glass. Two or more electrodes are submerged in the molten glass for heating the glass through the Joule effect when electric power is applied between the electrodes. The raw glass batch is supplied to, and floats upon, the upper surface of the molten glass, while the molten glass is removed at a submerged throat located in a sidewall or bottom of the melting chamber.
Various forms of refractory materials are used for holding molten glass in glass melting furnaces. Typical glass contact refractory materials for low alkali borosilicate glasses include chromic oxide, zircon and dense fused silica. The useful life of refractories for glass furnaces is primarily determined by the rate which the refractory material dissolves in the molten glass. Of the available refractory materials for use in low alkali borosilicate glass furnaces, chromic oxide has the longest life, lasting at least ten times longer than zircon, the next best refractory material, and up to 100 or more times longer than other refractory materials. Chromic oxide refractory, however, has a low electrical resistivity at temperatures encountered in glass melting furnaces, which low resistivity can cause problems when used in electric furnaces.
Alkali metals, usually sodium or potassium, are often added to glass as a flux to facilitate melting the glass and to lower the viscosity of the molten glass. However, alkali metals cause hot glass to have a low electrical resistivity. Glasses used for manufacturing electrical insulators and much of the glass used for manufacturing glass fibers for textiles, known as E-glass in the industry, typically have less than 1% alkali metal content. As a result, in a molten state, these glasses have a relatively high electrical resistivity compared to normal glass. A typical composition for such glass as set forth in TABLE 20-1 on page 375 of GLASS ENGINEERING HANDBOOK, Second Edition by E. B. Shand, is: SiO.sub.2 54.5%, Al.sub.2 O.sub.3 14.5%, CaO 22.0%, B.sub.2 O.sub.3 8.5%, Na.sub.2 O 0.5%, whereas U.S. Pat. No. 3,818,112 sets forth a typical composition of E-glass as follows: SiO.sub.2 54%, Al.sub.2 O.sub.3 14%, CaO 17.5%, MgO 4.5% and B.sub.2 O.sub.3 10%.
In the past, attempts to electrically melt glass having a high resistivity in a furnace formed from a chromic oxide refractory material have had limited success. The low resistivity of chromic oxide refractory material and the high resistivity of the molten glass cause a significant portion of the electrical current delivered to the furnace to flow through and heat the refractory rather than the glass. This causes rapid corrosion of the refractory. One solution to this problem is to use a refractory having a high resistivity at the temperature of the molten glass. However, zircon, the next best refractory material, dissolves in the glass much more rapidly than chromic oxide, and after dissolving in the glass at furnace temperatures it often recrystallizes from the glass as the temperature is reduced to working temperatures. Another solution is to use arc circuit electrodes as shown in U.S. Pat. No. 4,514,851, however, symmetrical power distribution within the bath is difficult to obtain, and the glass oxidation state may be difficult to control.
As shown in U.S. Pat. Nos. 3,806,621 and 3,818,112, and British Patent Specification No. 1 473 091, where electric power has been used in the past, the electrodes have been inserted through the bottom refractory, which is usually a highly resistive zircon refractory. The electrodes must be placed and energized to minimize the flow of current through the chromic oxide walls. That is, as pointed out in said U.S. Patents, in order to minimize electrical conduction through the sidewalls, first and second sets of electrodes are arranged through the bottom of the furnace with one set centrally disposed in the tank and the other surrounding the first. In a like manner, the British patent utilizes bottom mounted inner electrode means situated at or near the central region of the furnace, and a plurality of outer electrodes projecting upwardly through the bottom wall adjacent the peripheral wall and encircling the inner electrodes. Such arrangement is used to establish current paths through the molten body between the inner and outer electrode means while avoiding the application of any substantial voltage gradient between the outer electrodes and the peripheral wall.
When melting the glass with only electric power, the batch crust tends to trap the gases released by the melting batch materials, and the trapped gases lead to a reduced melting rate and to melting temperatures which are difficult to control. Thus, a major problem resided in the inability to obtain a controllable batch blanket because of gas bubbles.
It has been found that the use of batch electrodes, which are immersed through the batch crust, tends to eliminate the entrapment of gases in the blanket by preventing the formation of such a hard imprevious crust. The batch electrodes also improve the melting rate of the batch materials by releasing energy immediately beneath the batch crust. By positioning the batch electrodes in certain locations adjacent the crust, the heat transfer to the batch is optimized and the melting rate increased. Further, the use of batch electrodes allows a deeper furnace which is more suitable for producing high quality E-glass.